The present invention relates to multilayer ceramic wiring boards for providing various electronic circuits for use in electronic devices such as portable telephones, and to processes for producing such wiring boards.
With compact electronic devices such as portable telephones, it is conventional practice to arrange a plurality of circuit elements constituting the device into a multilayer ceramic wiring board, as integrated into a single chip and mount the wiring board on the main substrate.
As shown in FIG. 19, the multilayer ceramic wiring board comprises superposed ceramic layers 7 each having a plurality of circuit elements 71 formed on the surface thereof and providing an inductor or capacitor. Such circuit elements 71 are interconnected by a continuity channel (hereinafter referred to as a xe2x80x9cvia holexe2x80x9d) 73 extending through the ceramic layer 7 to provide an electric circuit such as a filter.
In producing the multilayer ceramic wiring board, sheets (hereinafter referred to as xe2x80x9cgreen sheetsxe2x80x9d) comprising a ceramic-incorporating material which contains a ceramic powder and a binder resin are prepared first by coating a surface of a plastic film with the ceramic-incorporating material in the form of a sheet and drying the material. A pattern of circuit elements in accordance with the design of the contemplated electronic circuit is subsequently printed on the surface of each green sheet using a conductive mixture material containing an electrically conductive powder and a binder resin. The printed green sheets are then placed over one another to prepare a laminate, which is thereafter pressed hot and thereby consolidated. The pressed laminate is further fired, and the fired laminate is finally cut into a plurality of multilayer ceramic wiring boards.
In recent years, it is required with increasing severity to compact electronic devices. In order to compact the multilayer ceramic wiring board itself, studies are underway to reduce the thickness of the ceramic layers 7 which are components of the wiring board.
As shown in section in FIG. 20, however, the boundary between the circuit element 71 and the ceramic layer 7 in the conventional multilayer ceramic wiring board is likely to become wavy or undulate in the region C indicated although the boundary should originally be straight, such that if the ceramic layer 7 is made thinner, the surface of the circuit element 71 will come into contact with the surface of other circuit element 72 or 72 in the wavy boundary region to make a short circuit.
Further as shown in FIGS. 21 and 22, it is likely that projections 95 will be formed on the surfaces of circuit elements 71 to extend toward other circuit elements 71. In the case where the ceramic layer 7 has a reduced thickness, the outer end of the projection 95 will come into contact with that of the projection 95 formed on the surface of other circuit element 71, giving rise to the problem of causing a short.
A first object of the present invention is to prevent formation of projections extending from the surfaces of the circuit elements on the multilayer ceramic wiring board. A second object of the invention is to prevent the waviness of the boundary between the circuit element and the ceramic layer in the multilayer ceramic wiring board.
To fulfill these objects, we have conducted intensive research and found that the formation of projections from the circuit elements on the conventional multilayer ceramic wiring board is attributable to the state of the ceramic powder dispersed over the surface of the green sheet and the particle size of the electrically conductive powder contained in the conductive mixture material, and that the wavy boundary between the circuit element and the ceramic layer is attributable to the state of the ceramic powder dispersed throughout the green sheet. With reference to FIG. 23, the surface of the conventional green sheet has interstices 82 formed between the ceramic particles 8 since the ceramic particles 8 are dispersed unevenly. The interstices 82 are filled with the binder resin or solvent. Further with reference to FIG. 24, the conductive mixture material contains the conductive powder 9 comprising particles of varying sizes. The mean particle size of the conductive powder 9 contained in the conductive mixture material was found to be approximately the same as that of the ceramic powder constituting the green sheet. When a pattern of circuit elements was printed on the surface of each green sheet 80 with the conductive mixture material 91 as shown in FIG. 25 in the conventional process for producing the conventional multilayer ceramic wiring board with use of conventional green sheets and the conventional conductive mixture material, it was found that conductive particles 9a of small size were trapped in interstices 82, 82, 82 formed in the surface of the green sheet 80 at the boundary (region D) between the green sheet 80 and the conductive mixture material 91 as shown in FIG. 26. The green sheets 80 each bearing the print of conductive mixture material were then placed over one another into a laminate, which was pressed hot, whereby the interstices in each green sheet were filled with conductive particles 9a of small size as shown in FIG. 27. The conductive particles 9a filling the interstices of the green sheets form projections on the surfaces of the circuit elements obtained by firing the pressed laminate.
The ceramic powder 8 was unevenly dispersed throughout the green sheet used for producing the conventional wiring board as shown in FIG. 26. Accordingly, when the laminate of printed green sheets 80 was pressed hot, compressive stress failed to uniformly act on each green sheet 80, consequently permitting waviness of the surface of the green sheet after hot pressing to undulate the boundary between the circuit element obtained by firing the laminate and the ceramic layer thereof. Especially when small circuit elements 72, 72 were present in the vicinity of the circuit element 71 as shown in FIG. 20, the compressive stress acting on the green sheet when the laminate was pressed hot involved great variations, consequently readily permitting waviness of the boundary between the ceramic layer and the circuit element.
To accomplish the first object, the prevent invention provides a multilayer ceramic wiring board prepared from a plurality of sheets formed from a ceramic-incorporating material containing a ceramic powder and a binder resin and each having a pattern of one or a plurality of circuit elements printed on a surface thereof with use of a conductive mixture material containing an electrically conductive powder and a binder resin, by placing the sheets over one another to form a laminate, and compressing and firing the laminate. The mean particle size Rs of the ceramic powder is not greater than the value of (Rdxe2x88x92"sgr"d) wherein Rd is the mean particle size of the conductive powder and "sgr"d is the standard deviation obtained when the distribution of the particle sizes of the conductive powder is expressed by a normal distribution function.
The present invention further provides a process for producing the multilayer ceramic wiring board which process has the steps of:
preparing a plurality of green sheets by coating a plastic film with a ceramic-incorporating material containing a ceramic powder and a binder resin and drying the ceramic-incorporating material,
forming a hole in at least one of the green sheets through the thickness thereof at least one portion of the sheet,
printing a pattern of one or a plurality of circuit elements on a surface of each of the green sheets with use of a conductive mixture material containing an electrically conductive powder and a binder resin,
placing over one another the printed green sheets on which the pattern of circuit elements is each printed according to the printing step to form a laminate,
compressing the laminate, and
firing the compressed laminate to harden the laminate,
the mean particle size Rs of the ceramic powder for use in the green sheet preparing step being not greater than the value of (Rdxe2x88x92"sgr"d) wherein Rd is the mean particle size of the conductive powder and "sgr"d is the standard deviation obtained when the distribution of the particle sizes of the conductive powder is expressed by a normal distribution function.
In the case of the multilayer ceramic wiring board of the invention and the process thereof for producing the wiring board, the distribution of particle sizes of the conductive powder contained in the conductive mixture material can be considered to be in conformity with a normal distribution function. In this case, the particle sizes of about 85% of the whole conductive powder contained in the conductive mixture material are greater than the mean particle size of the ceramic powder. Furthermore, the mean particle size of the conductive powder contained in the conductive mixture material is greater than that of the conductive powder contained in the conventional conductive mixture material. The conductive powder therefore encounters difficulty in entering the interstices formed in the surface of the green sheet in the pattern printing step and the compressing step. Thus, the conductive powder is less likely to fill the interstices in the surface of the green sheet, with the result that the formation of projections extending from the surfaces of circuit elements obtained by the firing step is prevented.
Stated more specifically, the pattern of one or a plurality of circuit elements printed on the surface of each green sheet constituting the laminate forms the circuit element or elements when the laminate is fired, and the value (Rd+2"sgr"d) of the conductive powder is not greater than {fraction (1/10)} of the value Wd which is the minimum line width of the circuit element or elements.
With this specific construction, the particle sizes of about 98% of the whole conductive powder contained in the conductive mixture material are not greater than {fraction (1/10)} of the minimum line width of the circuit element or elements and are sufficiently small relative to the minimum line width. Accordingly, the print of circuit element pattern formed by the printing step has diminished surface irregularities, making it possible to give the circuit element a line width as designed. As a result, no circuit noise occurs owing to the difference of the line width of the circuit element from the designed value.
Further stated more specifically, the value (Rd+2"sgr"d) of the conductive powder is not greater than 10 xcexcm, and is more preferably not greater than 5 xcexcm.
According to this feature, about 98% of the whole conductive powder contained in the conductive mixture material comprises particles of up to 10 xcexcm in size which are sufficiently small as compared with the line width (about 100 xcexcm) of usual circuit elements. As a result, the print of circuit element pattern obtained by the pattern printing step has diminished surface irregularities. Especially in the case where about 98% of the whole conductive powder contained in the conductive mixture material is up to 5 xcexcm in particle size, the particle sizes of the conductive powder are much smaller than the usual line width (about 100 xcexcm) of circuit elements, so that the print of circuit element pattern resulting from the pattern printing step has almost no surface irregularities. The circuit elements obtained by the firing step are therefore arranged at a spacing of designed value (so-called line pitch).
To accomplish the foregoing second object, the present invention provides another process for producing the multilayer ceramic wiring board which process has the steps of:
preparing a plurality of green sheets by coating a plastic film with a ceramic-incorporating material containing a ceramic powder and a binder resin and drying the ceramic-incorporating material,
a preliminary pressing step of compressing the green sheets obtained by the green sheet preparing step in the direction of the thickness thereof,
forming a hole in at least one of the green sheets through the thickness thereof at least one portion of the sheet,
printing a pattern of one or a plurality of circuit elements on a surface of each of the green sheets with use of a conductive mixture material containing an electrically conductive powder and a binder resin,
placing over one another the printed green sheets on which the pattern of circuit elements is each printed according to the printing step to form a laminate,
compressing the laminate, and
firing the compressed laminate to harden the laminate,
the mean particle size Rs of the ceramic powder for use in the green sheet preparing step being not greater than the value of (Rdxe2x88x92"sgr"d) wherein Rd is the mean particle size of the conductive powder and "sgr"d is the standard deviation obtained when the distribution of the particle sizes of the conductive powder is expressed by a normal distribution function.
The green sheets are compressed in the direction of thickness thereof by the preliminary pressing step of the process of the invention, so that the ceramic powder in the green sheets is made almost free from interstices. The preliminary pressing step further smooths the surface irregularities of the green sheets, rendering the surfaces of the green sheets substantially planar. Accordingly, the green sheets remain almost uncompressed by the compressing step to produce no waviness in their surfaces, whereby the boundary between each green sheet and the conductive mixture material is made approximately planar. As a result, the circuit elements resulting from the firing step have an approximately planar boundary with the ceramic layer.
The preliminary pressing step almost completely eliminates interstices from the ceramic powder in the green sheets, with the result that the surfaces of the green sheets are almost free from interstices. Furthermore, the distribution of particles sizes of the conductive powder contained in the conductive mixture can be considered to be in conformity with the normal distribution function. In this case, the particle sizes of about 85% of the whole conductive powder contained in the conductive mixture material are greater than the mean particle size of the ceramic powder. Furthermore, the mean particle size of the conductive powder contained in the conductive mixture material is greater than that of the conductive powder contained in the conventional conductive mixture material. The conductive powder therefore encounters difficulty in entering the interstices in the surface of the green sheet. This prevents the formation of projections on the surfaces of the circuit elements obtained by the firing step.
Stated more specifically, the green sheets resulting from the preliminary pressing step have an arithmetic average surface roughness Ra which is not greater than {fraction (1/20)} of the value Rs. The arithmetic average surface roughness is defined in JIS B 0601 and represented by Ra.
With this feature, the arithmetic average surface roughness Ra of the green sheets obtained by the preliminary pressing step is not greater than {fraction (1/20)} of the mean particle size Rs of the ceramic powder used in the green sheet preparing step, and the green sheets obtained from the preliminary pressing step have a substantially planar surface.
As described above, the multilayer ceramic wiring board of the invention and the process thereof for producing the wiring board are adapted to prevent formation of projections extending from the surfaces of circuit elements and also to prevent undulation of the circuit elements.